Wednesday, November 30, 2011

Bear in mind track is not set in stone and will likely shift before event occurs.

The models continue to shift northward, and because the GFS is moving towards the ECMWF solution, as are the ensembles, we have deemed it appropriate to make a rough sketch of a graphic. We are expecting a strong ridge of high pressure to hold in place on the East Coast and also keep much of the East warmer. This ridge will be pulling in warm, moist air from the Gulf of Mexico, creating an apt environment for some showers, depending on the moisture amount when the system comes through. To the west of the storm track, a cold blast of air may come down from portions of Canada in the wake of this storm. As this storm moves towards the north, a potential cold front extending from it will likely push away the warmer air and also help along the colder air. On the bottom side of the storm track, rain is widely expected thanks to the massive ridge in the East. This system will be grabbing the warm air and using it as moisture, which may help along the snowfall and rainfall totals in the end. For areas on the north side of this system, snow will occur. It is expected that the heaviest snowfall totals will occur right next to the track itself, where the moisture bands will be the tightest swirling around the low. This colder air that may also be in place would help the potential for snow as well.
The reason why this storm is not going south is because of the ridge and the cold air. It is a 'sandwich' situation, with two factors on the outside and the storm in the middle. The stronger component (ridge or cold air) will be the more determining factor for this storm. At this point, it appears that the ridge will be stronger and thus will push the storm farther west and north. If, for some reason, the models backtrack and decide that the ridge will be much weaker, then the storm track would likely short farther south. What is interesting about that is how the GEM model is following that potential of a stronger cold blast.
What may also be a factor is how fast the storm moves, and may play a bigger part than we may be noticing. The GEM has the storm track more south because that cold blast is coming out first instead of the storm as the storm moves slower and is thus pushed south. The ECMWF moves very fast, much faster than some other models. Thus, the storm comes first rather than the cold air. This would be a lose-lose situation, because less snow would fall and more rain would fall as the storm moves faster and is actually a bit north of the forecast graphic above.
We are consulting with other weather enthusiasts for their take on the storm track, so if you have anything to offer, drop a comment below- we'll respond to any questions you have.

Remember- this graphic is nowhere near set in stone, and was created to show current model guidance. The situation will change, and it looks like the situation should get itself intact going into the weekend.

GFS ensembles are forecasting brutally cold air to come down from the south in droves as Old Man Winter aims to make his presence known. Most of the ensembles are showing extremely cold air, while others show not as bad temperature anomalies but still well below average across the board. The 12th ensemble in the mix (counting from right to left) breaks the lowest this legend can go for below normal temperature anomalies. That's right, that ensemble member has temperatures forecasted to be at least 42 degrees below normal. That would certainly go below 0 degrees Fahrenheit. However, this is hour 240 of the forecast, considered long range. We will monitor this prospect however, and keep you informed nonetheless.

The Midwest remains in the potential line of fire for a major winter storm heading into the December 4-9 timeframe. The first image details the ECMWF's solution. The ECMWF continues to keep north, not budging at this time. Snowfall accumulations would easily rise above 6 inches in this scenario that the ECMWF is showing.
The second image is made up of the 12z NOGAPS model ensembles. At this time, pay attention to the models that are inside the black lines. Those are the models that are tracking this storm. The NOGAPS ensembles are keeping on the southern track at this time, which is somewhat surprising considering the NOGAPS itself is on a more northerly track at this time. However, because the models are not too good at this time, we are more angled to trust the ensembles.
The third image details the NCEP ensembles. Again, pay attention to the tracks inside the black lines. The NCEP ensembles are following the GFS, but at this time appear to be a tad more southerly than the actual GFS. We are again more apt to go to the ensembles at this time of model mayhem.
The fourth image is comprised of the 12 GFS and 12z GEM models. The GEM/GFS appear to be on a similar track until Illinois, when the GEM dips south as the GFS continues a northeast track.
All that said, here's the current camps of north track and south track.
The north track is above North Illinois, while the south track is below North Illinois.
(ENS. stands for ensembles)

Ratio of North Camp to South Camp is 5:6 out of 11 models in favor of the South Track.

It does look like a ridge in the east and a cold outbreak in the North Plains will be two key factors in this storm track. The ECMWF pushes the warmer air farther westward, thus bringing the storm track in the same direction. The 12z GFS is more forceful with the cold air and keeps the storm track down south. However, the GFS has started to trend north, as illustrated in the below picture:

Past 4 cycles of GFS (see color key on bottom right)

The GFS, as you can see, has been very stable the last couple runs but has trended north for one run. We are waiting for the 0z run to see if this change will continue or if this run was a fluke.

We are thinking that a blend of the ECMWF/GFS is a good idea at this point, but definitely leaning with the ensembles as the models struggle in some spots.

If you want to see the models for yourselves, check out our updated Weather Models Page, now housing 144 links for 25 models.

Various Models are indicating the potential for a snowstorm in the December 4-9 timeframe. This system would come as a low pressure system cut off from the jet stream that would hang around the Southwest for a day or two. After that, this system would supposedly move to the general NE/NNE (depending on model discretion) and affect the Midwest.
The models are very spread about right now, and they ought to be- it's winter transition time, which is never easy for the weather models. We do have some disorganized 'camps' set up: the north track or the south track The north track involves the ECMWF solution shown above, while the south track is more like the NOGAPS track. Here's the gist of the models right now:

North Track: ECMWF, GEM, DGEX, JMASouth Track: GFS, NOGAPS

(Note: JMA and DGEX are not very reliable models)
This will likely come down to the eleventh hour, so to speak, so let's check in on the ensembles, which are much more reliable and consistent than the models themselves at this point. Using the same definitions for the north and south tracks, here's the ensembles take.

North Track: ECMWF EnsemblesSouth Track: GEM Ensembles, GFS Ensembles

We are not going to even try to call this one, as the models swerve like a car going down a very icy road.

There is potential for snowfall rates of up to 1 inch an hour as a cold-core upper level low circles in Kentucky/Tennessee. A cold-core upper level low (ULL) system is basically a low pressure system with cold air in the center of it. That's how snow is possible in Tennessee into Mississippi, Alabama. It is not out of the question that accumulating snows may fall in the aforementioned areas. As this cold core ULL eventually moves off, our concern will shift to another threat that may involve a snow threat for Wisconsin later on in the week.

Sunday, November 27, 2011

Recap of LRC:
-Pattern setting up between Oct. 1-Nov. 10 every year.
-Certain atmospheric features will repeat approximately 30-60 days after it occurs if this feature is within the timeframe mentioned above.
-Pattern repeats through winter, spring, into summer.
-Pattern is never the same each year.

Note about this year's LRC:
-Each cycle is 46 days long. That is, each system below would theoretically repeat every 46 days.

October 19

SYSTEM 1: October 19, 2011

An Upper Level Low was cut off from the jet stream on October 19th. The piece of energy stayed in place, putting precipitation over the same areas for a couple days, before it was nudged north back into the jet stream. Over this time period, the equivalent of around 6 inches of snow (that originally fell as rain on October 19) fell over the Michigan area/ Ohio Valley. This same system is currently repeating at the time of posting. Because this is happening, the chances of the next system occurring are rising.

November 9

SYSTEM 2: November 9, 2011

A possible Panhandle Hooker storm came up into the Midwest/Great Lakes region, putting down impressive precipitation totals. In the 3 days pan that was measured, as much as 1.97 inches of rain was recorded in the region most affected. This would easily equate to 20 inches of snow. It's pretty crazy to think that 20 inches of snow might fall in one area every 46 days. But if the LRC pans out, the atmosphere might be able to pull this feat off. The chances of this are increasing as the 1st system is panning out. If the 46 days are true, then this system could be coming back around December 28th- right after Christmas.

The 12z GFS is forecasting a devastating snowstorm for Michigan on November 30. While the image above encompasses 5 days from now, we have investigated and these high totals do come from the November 30th.
The interesting thing is how the NAM does not bring this solution to light. It appears that the models are divided into two camps:

Snowstorm Camp: GFS, ECMWF, JMA, UKMET

No Snowstorm Camp: NAM

(GEM has mostly rain but back-end snow, will not put in category)

I would say that there ought to be a snowstorm, but may not be as severe as some models are projecting.

Saturday, November 26, 2011

The unreliable JMA weather model is forecasting Michigan to be demolished with over 2 feet of snow 96 hours away from today. Everything above the pink line is snow. As you can see, this JMA model is ridiculously strong and beyond any chance it will verify. It is, however, fun to look at.

Using HPC's 5-day rainfall forecasts, we have put together a 5-day rainfall forecast for the US.
We do believe there is potential for over 4 inches of rain to fall in the Ohio Valley and parts of the Southeast. It looks like the instigator for this rainfall will be the cold front currently progressing east over the East US. After that, another factor will be a piece of energy associated with the front moving north, deepening and being cut off from the jet stream for a little while, leaving rain to move over the same areas for a while.

The cold front currently pushing through the Midwest will continue to do so through the afternoon. The above image is for 4:00 pm CDT as the cold front will continue to push through. Behind this, colder air will sweep in as the stage is set for cooler days to begin their takeover.
At this point, it is unlikely that another huge warm burst will occur in the next week, signaling that the transition into winter is beginning. Meteorological winter does start on December 1st, so I guess it is about time for winter to start.
The rain this cold front will be putting down is fairly large- check out the forecasted total precipitation from the same model as the image above.

It looks like Chicago, IL and possibly Gary, IN could be within this heaviest precipitation after the front moves through.

We are predicting an area of cold air to produce some snow, possibly heavy, in the southern US.
This forecast is NAM-biased. The only reason it is is because the NAM is showing this amount of snow shown while the GFS does show an area where snow could fall, but for some reason shows no accumulation where snow is clearly falling.
I never had a good feeling about this system and still don't, but I would rather have put out a warning and be wrong than not put out a warning and be drastically wrong.
That said, use this map with caution as this may be subject to drastic changes.

It appears that the cold front moving east will ignite an area of low pressure out by the Tennessee region. This area of low pressure will be cut-off from the jet stream, thus keeping the system in place. When the low does form, the 700mb relative humidity values will also deepen within the low, and will thus increase chances for precipitation.

This low will stay in position until a weak piece of energy will force the low northward and back into the jet stream. While the low is in position, however, models are predicting a piece of cooler air coming from the cold front to be cut off from the original cold air that the cold front is pulling. This colder air will turn counter-clockwise around the low and may produce some wet snow. Up to 4 inches of snow may be possible in Mississippi if this turns out as is projected.

Friday, November 25, 2011

The Arctic Oscillation is an oscillating index based in the Arctic Circle region.
When the Arctic Oscillation is in a positive phase, a vortex in the upper regions of the Northern Hemisphere is strong and locks in cold air to keep it up north. This keeps the cold air up north as stated, and does not let the US get colder air.

When the Arctic Oscillation is in a negative phase, the vortex weakens, and the cold air locked in releases itself to more southerly regions of the North Hemisphere, typically affecting North America.

The AO can be a key player in winters when determining cold weather, and is an index that should be watched closely.

The LRC (Lezak's Recurring Cycle) is a pattern eagerly followed by many meteorologists and can be a key instrument in determining a winter forecast. Here are the guidelines:

-Pattern sets up between October 1 and November 10.-Between this time, a certain atmospheric feature occurs that will reoccur over the winter, spring and into summer over a 40-60 day cycle.-Pattern is never the exact same each year.

So, as an example, let's say a huge rainstorm came through the Ohio Valley on October 17th. Then, on November 30, another huge rainstorm came through the Ohio Valley. This may be considered a part of the LRC, if atmospheric conditions (similar strength, effects on atmosphere) agree with the October 17th storm.

The MJO, a strong influence of weather in the winter season, is not looking favorable for the next several days for the US. The GFS and ensembles have the MJO moving into Phases 3 and 4. Phases 3 and 4 would mean the following effects for the US:

1. Warm for Much of US2. Cool for SE in Phase 4, Cool for SW in Phase 3.3. Dry for Midwest, Ohio Valley, Upper North US for both Phases4. Wet in West US, Southeast in Phase 3.5. Wet in NW, NE in Phase 4.

Thursday, November 24, 2011

Today's new 12z GFS is predicting colder weather for the US in the long range (11-15 days out), falling into line with the potential for teleconnections to move into a better weather pattern more conductive for snow. The GFS has been having some trouble lately, so we will see if the GFS continues with this pattern before making an assumption about it.

The teleconnections are looking much better than they have been looking and may be hinting that this winter pattern change is closer than we think. Above are 3 important indices that are commonly tracked and do affect the weather. We have labeled what is good for snow and cold people in each image. Click on the following links for explanations of the indices:PNA (Pacific North American Pattern) --- NAO (North Atlantic Oscillation) --- AO (Arctic Oscillation)

PNA
The PNA has recently been in a strong negative phase, leading to unfavorable chances for snow and cold. However, in the above images, we see the majority of ensemble members are trending towards at least a weak positive phase already in progress but most likely becoming prominent just before November 30 or so. If the PNA is to go positive, chances for snow and cold would rise.

AO
The AO has been in a positive phase most recently. When it is in a positive phase, a vortex around the North Pole strengthens and literally locks in cold air, forcing it to stay up north. However, if the AO is in a negative phase, this vortex weakens and the cold air can escape south, sometimes into the US. The ensemble members forecasting for the AO indicate equal chances for repeating positive spikes or a downward trend towards negative territory. We will have to watch this closely to see what happens.

NAO
The NAO has recently also been in positive phase, not a good sign for those who like snow and cold. The ensemble members indicate there is a very substantial potential for the NAO to go negative, increasing chances for snow and cold. The AO and NAO are closely related, so if one goes negative it is possible the other will as well.

In the coming days we will be seeing a variety of things. One of them will be seeing the ridge of high pressure in the West dissipate and instead be replaced with areas of below average temperatures close to the West Coast. This may be indicative of some storm systems washing shore that would not be able to pump temperatures up. Another big factor that will enhance this dissipating ridge is the potential for another ridge of high pressure to form offshore the US and move towards Alaska. This ridge would therefore defeat the Aleutian semi-permament Low and would have potential to flip things around. This ridge would most likely lead to a trough in the West and a possible ridge in the East if that's how the atmosphere wants to play its games.

Winter has not been slow to arrive, even if you think it has. The reason you probably think winter is starting slow is because of the October snowstorm in the Northeast. Let me remind you how that storm was a VERY VERY LUCKY break for the Northeast and has a very low chance of happening anytime soon under normal conditions.

Anyways, this is a briefing on how winter is moving along.
To be honest, it hasn't been starting slow. We recorded last year O'Hare Airport, Chicago's first snowfall came on December 12th, and they ended up with a historic snowstorm. Just a little bit to tell you.

The La Nina has recently been rumored to be unusually weaker than last year. Well of course it's weaker- it was always expected to be weaker than last year, so the effects will not be just as strong. This leaves more space for other variables to intervene, which they have been doing at the snow lover's disadvantage. At this point in time, we are seeing how some indices will be moving towards better waters:

(Green text is good for snow lovers, red text is bad, yellow text could go either way.)

The PNA will be moving towards positive territory. The NAO will be moving towards more neutral territory rather than positive territory.The EPO will be dipping into negative territory, then staying around neutral territory.The AO will be sliding towards neutral territory, but then rebounding into positive territory.The La Nina should sustain itself or move slightly more negative over the winter.

The big question, of course, is when will winter start to act like winter??
We have gotten some indications that the Arctic Oscillation (AO) will be tanking at some point. We cannot independently confirm this, but we are looking for the certain indices that may be looking at this potential.
Recently, the ECMWF received an upgrade that some have indicated are showing signs of improvement in the model. Again, we cannot independently confirm this, but the long range ECMWF does show the Aleutian Low in Alaska breaking down (see what that does by clicking here), which may give the US a little taste of winter. The teleconnections are changing, so winter may be on its way!

Wednesday, November 23, 2011

Aleutian Low Example
(The Low Pressure system in Alaska is the Aleutian Low)

12z ECMWF Hour 240

In the top image, you can see the strong low pressure system located just around Alaska. This is called the Aleutian Low. This low pressure system is semi-permanent, meaning it can stay in that area for long periods of time and comes back frequently. When this happens, a consequence is that a ridge (high pressure system) forms in the West US, usually leading to some warmer air over the country. However, if this Aleutian Low breaks and dissipates, a high pressure can form over Alaska and lead to a low pressure forming over the West US, which can lead into colder weather for the country.
Are we saying this will happen? No. Are we saying it could be an indicator that the weather pattern may be changing soon? Possibly. We will have to wait and see at this point.

This Thanksgiving looks to be a quiet one across the country. Sunny skies are widely expected across the Eastern US, while some more humid air may provoke some showers and clouds as moisture from Tropical Storm Kenneth enters the country. A system will be moving onshore the Northwest region, so it can be expected some showers will be present as the day goes on. The Central US will likely have sunny skies.
Temperatures are expected to be above normal for the country.

The GFS is hinting that Mississippi, Alabama, Louisiana could be in for some snow going into the November 29th timeframe. This does have potential to play out, but it may be slim. For instance, check out the factor that will be inducing snow below

GFS 6z forecast (Anything inside red circle may be snow)

The GFS says a cold pool of air will swirl around a cutoff low (low pressure that has been cut off from the jet stream and sits nearly motionless) and provide snow for the Southern US. It is entirely possible that this does not happen, and I am not too optimistic about it. We will have more information soon.

The GFS and ECMWF models are hinting at a storm system over the Midwest/Ohio Valley in the very late November time frame. The GFS believes areas within the two aforementioned regions could receive some light snow, while the ECMWF takes the low farther north into Illinois where some snow would likely be found in Wisconsin. Temperature anomalies are suggesting the potential for a major cool down in the Southeast US into the Midwest and Plains, with temperature anomalies as low as under 10 degrees below normal.
The teleconnections are less impressed by this prospect. They are offering a small chance for some snow and cold chances, particularly in or just after the November 30th timeframe. Today's 12z ECMWF has indicated that the PNA index, so stubbornly negative for a long time now, thus preventing cold and snow, may turn positive for a couple days while this storm makes its way across the Ohio Valley and Midwest. However, the ECMWF also keeps the NAO, WPO and AO indices positive, which are bad signs for snow lovers. All in all, it's up to the teleconnections. Models are very prone to have trouble in the long range, so don't take this as gospel.

On a side note, the Accuweather Updated Winter Forecast will be released December 1st. From what i've heard, some changes (possibly bad for snow lovers) are in the works. I stand firm by my forecast and believe any changes that may be made that would result in a shortened or less-harsh winter for the Midwest is incorrect.

There is evidence that, in the last decade of temperature observations, global warming has been slowing in the United States.
We are not nearly joking on this matter, and we do have evidence to prove it. Below are some images from the National Climatic Data Center, or NCDC. The first image is of the average mean temperature from 2001-2011 with regional trends.

We can clearly see how much of the US has been slowing in global warming, even to the point of cooling down. The charts above the regions depict trends for a more in-depth view of how the temperatures have been sliding in the past decade.
What does all of this mean? It means that there is now solid evidence that global warming may have taken quite a hit, as this cooling indication is showing how winters are colder and summers are cooler over the last decade. If you want more images with regions, you can go to the NCDC's website.

The GFS model, based on the last several runs, appears to be easing out of the likelihood of a nosediving Arctic Oscillation. If the Arctic Oscillation were to go negative, cold air would start coming to the US. However, if the pattern keeps positive, winter may be delayed yet again.

Monday, November 21, 2011

There are those thinking that winter is lost because the East remains warm and is projected to for a while onwards. But here's the thing- while we sit in above normal temperatures down in the Lower 48, a positive Arctic Oscillation is literally forcing the cold air to stay up way north in Canada.

The Arctic Oscillation, when in positive, contains a vortex that spins around the Arctic Circle region and bottles up the very cold air. This cold air stays up there until this vortex releases its grip. This releasing is the Arctic Oscillation's negative phase. After the vortex weakens, the brutally cold air squirms its way south and eventually hits the US.

The recent values of the AO have been largely positive. There was a point when the AO blipped negative, but no real effects were felt. Notice how far positive the ensembles take the AO. This means that the vortex would increase in strength, continuing to trap the cold air. Beyond that point, the models struggle. Recent GFS runs have been pointing to a nosedive, but seeing more ensembles provides a bigger possibility of a wider range of options.

There are 2 options at this point in time:

Option A) The AO goes on the GFS trend of a nosedive towards negative territory and goes into negative territory. Cold air bottled up for a long time is released southward, sending a chill throughout the US and possibly some snow. The winter pattern likely gets into gear at some point soon before/after.

Option B) A few ensembles predictions of another positive spike (following the first one in bold red in the image above) validates. Cold air remains situated in the far north, and the US remains mild. Winter is delayed until around Christmas (possibly).

If we had to give a guess into which option would turn out at this time, we actually wouldn't be sure.
The Pacific North American pattern (PNA) is forecasted by 2 major agencies. The NCEP agency is forecasting the PNA to go positive at the time of this nosedive projection, which would be very good for snow lovers in the US. However, the ESRL agency is forecasting a strong negative spike, which would be a 'more of the same' deal in terms of warmth in the US.

Sunday, November 20, 2011

We broke the 300,000 page view mark this evening, only 20 months after this blog started business!
Thanks to anyone and everyone out there who has supported us through the months, and we look forward to blogging about weather for years to come!

The 12z GFS, which had been forecasting the Arctic Oscillation to extend its raging positive phase for the Arctic Oscillation, has been rendered void by the new 18z GFS as the 18z GFS has resumed the trend of nosediving the AO. You can see the marked areas of the 12z GFS and 18z GFS. We will continue watching the forecast, but this is the development as we see it right now.

Due to the overwhelming majority voting 'yes' in our poll asking if you would like a sneak peek of winter 2012-2013, we will be releasing one in the next several days.
Keep in mind it has a fair chance of being inaccurate, as we are over a year away.

Today's new GFS has come in with a horrible concoction of teleconnections. We have edited the above map showing the Arctic Oscillation forecast from the past 4 GFS runs. The PNA is forecast to be negative, and the MJO will be moving through unfavorable phases.
All we will say at this point in time is to closely watch the 18z GFS. It will either validate this new forecast or void it.

Table 1: summary of Pacific and North American climate anomalies associated with extreme phases of the PDO.

climate anomalies

Warm Phase PDO

Cool Phase PDO

Ocean surface temperatures in the northeastern and tropical Pacific

Above average

Below average

October-March northwestern North American air temperatures

Above average

Below average

October-March Southeastern US air temperatures

Below average

Above average

October-March southern US/Northern Mexico precipitation

Above average

Below average

October-March Northwestern North America and Great Lakes precipitation

Below average

Above average

Northwestern North American spring time snow pack and water year (October-September) stream flow

Below average

Above average

Winter and spring time flood risk in the Pacific Northwest

Below average

Above average

Joe Bastardi, former Chief Long Range Meteorologist at Accuweather, has put up a message indicating that a +AMO/-PDO would help the potential for the theory of a major trough to hit the East/North US going into the last few days of November. You can see the differences between a warm PDO and negative PDO in the columns above, thanks to the University of Washington.
We are currently in a Cold PDO phase and a warm AMO phase. These indices change on a multidecadal basis- every couple decades. Since the mid 1990s, the AMO has been on a mainly positive streak. However, it is less positive than last year. During the warm phase of the AMO, temperatures across North America are above average, as shown by this map below thanks to Intellicast.

Since we are in a less positive phase than last year, it is possible that global temperatures may be slightly cooled from last year, in addition to low sunspots.
We will have more information on the trough as we come closer to the date.

Saturday, November 19, 2011

There is a very eerie similarity we have discovered in today's new 0z GFS. We have been closely following a pattern called the LRC (Lezak Recurring Cycle). Here's the gist of it:

-Observation period between October 1-November 10.
-Look for features present in weather over period.
-After November 10, look for repetition in weather from observation period to present time.
-Pattern different every year.
-Pattern repeats on 40-60 day cycle through winter, into spring.

This year, a dominant feature was a strong trough of low pressure in the Ohio Valley. See the below example from October 19th.

We see a strong low pressure system in the Midwest into Ohio Valley. A ridge was present in the west, holding up the jet stream in that area. Now see the forecast from the freshly-printed 0z GFS for the final days of November.

Look at just how similar this is: A strong low pressure system sets up in almost the exact same place as October 19th. A high pressure system sets up in the west US. The only difference is a low pressure system present just offshore the West Coast. However, we prefer to focus on the presence of a trough in the Midwest.

Now, according to LRC 'guidelines', the pattern typically does not appear this soon after it happens. If this was to verify, the cycle would happen every 39 days. That would be an extremely short cycle. However, seeing as how this is a very similar forecast to what happened on the 19th, it is possible that this LRC will turn out that way. Let's not get ahead of ourselves- this forecast is for November 27th.
It is definitely something to watch, though.

The past couple GFS model runs have shown a true nosedive in the Arctic Oscillation forecast going into the late November period. The thing about it is, since the GFS has been indicating a nosedive for more than 3 runs, we like to call that a 'trend'. More interesting is how this Arctic Oscillation nosedive occurs as today's 12z ECMWF predicts a massive trough to cross the Upper Midwest as indicated by the 500mb chart just above. This ECMWF model, should it verify, would likely give the Upper Midwest snow and possibly some intense thunderstorms in the south. It's hard to tell since this system is projected to be very strong. The strength of the storm only makes it more volatile.
Since the Arctic Oscillation is very closely related to the North Atlantic Oscillation, it is possible that the NAO may go negative if the AO does as well.

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